<!--
    @license
    Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
    This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
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<!--
`core-a11y-keys` provides a normalized interface for processing keyboard commands that pertain to [WAI-ARIA best
practices](http://www.w3.org/TR/wai-aria-practices/#kbd_general_binding). The element takes care of browser differences
with respect to Keyboard events and uses an expressive syntax to filter key presses.

Use the `keys` attribute to express what combination of keys will trigger the event to fire.

Use the `target` attribute to set up event handlers on a specific node.
The `keys-pressed` event will fire when one of the key combinations set with the `keys` attribute is pressed.

Example:

This element will call `arrowHandler` on all arrow keys:

    <core-a11y-keys target="{{}}" keys="up down left right" on-keys-pressed="{{arrowHandler}}"></core-a11y-keys>

Keys Syntax:

The `keys` attribute can accepts a space seprated, `+` concatenated set of modifier keys and some common keyboard keys.

The common keys are `a-z`, `0-9` (top row and number pad), `*` (shift 8 and number pad), `F1-F12`, `Page Up`, `Page
Down`, `Left Arrow`, `Right Arrow`, `Down Arrow`, `Up Arrow`, `Home`, `End`, `Escape`, `Space`, `Tab`, and `Enter` keys.

The modifier keys are `Shift`, `Control`, and `Alt`.

All keys are expected to be lowercase and shortened:
`Left Arrow` is `left`, `Page Down` is `pagedown`, `Control` is `ctrl`, `F1` is `f1`, `Escape` is `esc` etc.

Keys Syntax Example:

Given the `keys` attribute value "ctrl+shift+f7 up pagedown esc space alt+m", the `<core-a11y-keys>` element will send
the `keys-pressed` event if any of the follow key combos are pressed: Control and Shift and F7 keys, Up Arrow key, Page
Down key, Escape key, Space key, Alt and M key.

Slider Example:

The following is an example of the set of keys that fulfil the WAI-ARIA "slider" role [best
practices](http://www.w3.org/TR/wai-aria-practices/#slider):

    <core-a11y-keys target="{{}}" keys="left pagedown down" on-keys-pressed="{{decrement}}"></core-a11y-keys>
    <core-a11y-keys target="{{}}" keys="right pageup up" on-keys-pressed="{{increment}}"></core-a11y-keys>
    <core-a11y-keys target="{{}}" keys="home" on-keys-pressed="{{setMin}}"></core-a11y-keys>
    <core-a11y-keys target="{{}}" keys="end" on-keys-pressed="{{setMax}}"></core-a11y-keys>

The `increment` function will move the slider a set amount toward the maximum value.
The `decrement` function will move the slider a set amount toward the minimum value.
The `setMin` function will move the slider to the minimum value.
The `setMax` function will move the slider to the maximum value.

Keys Syntax Grammar:

[EBNF](http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form) Grammar of the `keys` attribute.

    modifier = "shift" | "ctrl" | "alt";
    ascii = ? /[a-z0-9]/ ? ;
    fnkey = ? f1 through f12 ? ;
    arrow = "up" | "down" | "left" | "right" ;
    key = "tab" | "esc" | "space" | "*" | "pageup" | "pagedown" | "home" | "end" | arrow | ascii | fnkey ;
    keycombo = { modifier, "+" }, key ;
    keys = keycombo, { " ", keycombo } ;

@group Core Elements
@element core-a11y-keys
@homepage github.io
-->

<link rel="import" href="../polymer/polymer.html">

<style shim-shadowdom>
  html /deep/ core-a11y-keys {
    display: none;
  }
</style>

<polymer-element name="core-a11y-keys">
<script src="core-a11y-keys.js"></script>
</polymer-element>
